Abstract

Small populations easily become extinct (Goodman, 1987). Small populations may also be affected by genetic drift and inbreeding (Schonewald Cox et al, 1983), although the importance of these latter effects on population dynamics is less clear (Ehrlich, 1983; Shaffer, 1987; see also chapter 4). The adverse demographic and genetic processes in small populations are both counteracted by immigration. Such immigration should keep the density over a certain threshold level (Brown & Kodric–Brown, 1977) or supply at least one reproducing individual per generation (Schonewald–Cox et al., 1983) to circumvent extinction or loss of genetic variability. However, immigration of foreign competing or predating species may have negative effects on local and circumscribed communities (Helle & Järvinen, 1986).

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